1. Technical Field
The present disclosure relates to a method and apparatus for manipulating body tissue or deploying surgical fasteners into body tissue, and, in particular, to a method and apparatus for determining parameters of the motion of a firing rod in a surgical instrument based on an electric field associated with the firing rod.
2. Background of Related Art
Current surgical instruments typically require 10-60 pounds of manual hand force to clamp body tissue and deploy surgical fasteners in body tissue. Repeated use of these surgical instruments can cause fatigue in a surgeon's hand. Powered surgical instruments were developed to, among other reasons, reduce or eliminate this fatigue. These powered surgical instruments include gas-powered pneumatic staplers, which implant surgical fasteners into body tissue. Certain of these instruments include a pressurized gas supply coupled to a firing mechanism and a trigger mechanism. The trigger mechanism, when depressed, releases pressurized gas, which, in turn, applies force to the firing mechanism to deploy a surgical fastener into body tissue.
Powered surgical instruments also include motor-powered surgical instruments. These surgical instruments include powered surgical staplers with motors that activate staple firing mechanisms. Typically, the motors are rotary motors mechanically coupled to a lead screw so that the motor can cause the lead screw to rotate. The lead screw has a continuous helical thread machined on its outer surface along its length (similar to the thread on a bolt). Threaded onto the lead screw is a nut with corresponding helical threads. The nut, however, does not rotate with the lead screw. In this configuration, when the lead screw is rotated by the motor, the nut is driven in a linear direction. The nut, in turn, drives a mechanism for manipulating body tissue or deploying a surgical fastener into body tissue. Alternatively, the lead screw is replaced by a firing rod with helical threads on its outside surface and the nut is replaced by a drive tube with corresponding threads on its inside surface (as described below). In this configuration, the motor rotates the drive tube and the drive tube, in turn, drives the firing rod in a linear direction. The firing rod, in turn, drives the mechanism for manipulating body tissue or deploying a surgical fastener into body tissue.
In some surgical instruments, a controller controls the motion of the firing mechanism (e.g., the nut or the firing rod) based on feedback from sensors that sense parameters of the linear motion of the firing mechanism (e.g., velocity). A conventional method of sensing parameters associated with the motion of a firing rod is to use a rotational sensor mechanically coupled to the rotary motor or the drive tube that drives the firing rod.
A typical rotational sensor includes an encoder wheel coupled to the drive shaft of the rotary motor (or the drive tube), a light generator, and an optical reader (e.g., photo interrupter). The encoder wheel includes a plurality of slits disposed around its outer edge and rotates with the drive shaft. The outer edge of the encoder wheel is disposed between the light generator and the optical reader so that the light generator emits a light beam through the slits to the optical reader. In other words, the light beam is interrupted by the encoder wheel as the drive shaft rotates. The optical reader determines the number of interruptions in the light beam and rate of interruptions and transmits these measurements to a processor, which determines the speed of the drive shaft. The processor then uses the speed of the drive shaft to calculate the linear velocity of the actuator (e.g., firing rod) mechanically coupled to the drive shaft.
Rotational sensors as well as other existing types of sensors, however, contribute in a significant way to the size, length, diameter, weight, and complexity of a surgical instrument. In addition, many of these sensors increase mechanical wear within the surgical instrument because the sensors mechanically interact with or repeatedly make physical contact with components of the surgical instrument. Therefore, there is a continual need for surgical instruments having sensors that reduce mechanical wear (for increased reliability), that reduce the complexity of the design of the surgical instrument (for reduced fabrication costs), and that reduce the size, length, diameter, and weight of the surgical instrument (for increased maneuverability during laparoscopic and endoscopic procedures).